Entanglement Between Photons that have Never Coexisted

I have posted many times that entanglement between photons that have never existed at the same time is theoretically possible under QM. Now an experiment has been performed that demonstrates this. The technique uses entanglement swapping.

Photons 1 and 2 are created as an entangled pair. The polarization of photon 1 is measured, so it no longer exists. Later, photons 3 and 4 are created as an entangled pair. Even later, the entanglement of photon 2 is swapped with photon 3. This means that photon 4 is now entangled with photon 1, even though photon 1 no longer exists!

Experimental realization of that setup shows that photons 1 and 4 violate a Bell Inequality, demonstrating their entanglement. Whoever said QM was a "strange theory" was certainly not exaggerating! A future context affects the past (since the decision to entangle 1 and 4 is made after 4 is detected) !!

"The role of the timing and order of quantum measurements is not just a fundamental question of quantum mechanics, but also a puzzling one. Any part of a quantum system that has finished evolving, can be measured immediately or saved for later, without affecting the final results, regardless of the continued evolution of the rest of the system. In addition, the non-locality of quantum mechanics, as manifested by entanglement, does not apply only to particles with spatial separation, but also with temporal separation. Here we demonstrate these principles by generating and fully characterizing an entangled pair of photons that never coexisted. Using entanglement swapping between two temporally separated photon pairs we entangle one photon from the first pair with another photon from the second pair. The first photon was detected even before the other was created. The observed quantum correlations manifest the non-locality of quantum mechanics in spacetime."

Photons 1 and 2 are created as an entangled pair. The polarization of photon 1 is measured, so it no longer exists. Later, photons 3 and 4 are created as an entangled pair. Even later, the entanglement of photon 2 is swapped with photon 3. This means that photon 4 is now entangled with photon 1, even though photon 1 no longer exists!

Can you set up an entanglement with something that is already measured/known? If photon 1 is measured and known, then so is photon 2. Can you then entangle photon 3 with photon 2 when photon 2 is already known/measured? That may contradict the definition of entanglement.

Can you set up an entanglement with something that is already measured/known? If photon 1 is measured and known, then so is photon 2. Can you then entangle photon 3 with photon 2 when photon 2 is already known/measured? That may contradict the definition of entanglement.

Yes, this is no problem. It is just normal entanglement swapping which does not depend on a particular time ordering of detections and the decision to swap. In the total context, photon 2 is entangled even after photon 1 has been detected. The total context can span both space and time, with the unusual feature that the time context can include the future.

Yes, this is no problem. It is just normal entanglement swapping which does not depend on a particular time ordering of detections and the decision to swap. In the total context, photon 2 is entangled even after photon 1 has been detected. The total context can span both space and time, with the unusual feature that the time context can include the future.

Are you saying that all the photons, 1, 2, 3, and 4, were all entangled together before they were separated by pairs and before photon 1 was measured?

Photons 1 and 2 are created as an entangled pair. The polarization of photon 1 is measured, so it no longer exists. Later, photons 3 and 4 are created as an entangled pair. Even later, the entanglement of photon 2 is swapped with photon 3. This means that photon 4 is now entangled with photon 1, even though photon 1 no longer exists!

Experimental realization of that setup shows that photons 1 and 4 violate a Bell Inequality, demonstrating their entanglement.

Great post Dr. Chinese on an interesting phenomena. Thanks for sharing another paper.

From the paper:

"Although one photon is measured even before the other is created, full quantum correlations were observed by measuring the density matrix of the two photons, conditioned on the result of the projecting measurement."

b) how is a density matrix formed between 1 & 4 - given that - 1 no longer exists?

Seems like the above is explainable if we assume that a third "entity" exists:

that stores/holds the entanglement states and transfers this state onto "new" photons when an interaction happens.

Are you saying that all the photons, 1, 2, 3, and 4, were all entangled together before they were separated by pairs and before photon 1 was measured?

The photon pairs 1&2 and 3&4 were entangled due to their respective BBO crystal, and all 4 are never entangled in the same manner as 4 photon entanglement. Once 2&3 get projected into a specific state at a beam splitter, it throws 1&4 into an entangled state.

Note that the entangled state of 1&4 can transcend the normal limits of spacetime. For example, 1&4 could be separated in such a way that no classical signal could move from one to the other. This particular twist was not used in this experiment, but it is possible. Instead, we had the situation where 1&4 don't coexist in any reference frame. That distinction is not relevant to the QM predicted results. But that obviously wouldn't fly in a classical regime - how can you entangle a particle that doesn't exist after the fact?

Should this impress me? Perhaps I don't know enough about entanglement to really grasp the impact of this.

Ha, not sure what it takes to impress you. But yes, this is realization of an important variation in experiments the Zeilinger et al group has done previously. They showed entanglement of particles that had never interacted in their past light cones, although they did exist at the same time. This goes further by showing that no variation on time ordering changes the outcome of the experiment.

A further refinement would be to combine both variations to demonstrate entanglement of particles which not only have never existed in a common light cone, but also never existed at the same time.

Note that the entangled state of 1&4 can transcend the normal limits of spacetime. For example, 1&4 could be separated in such a way that no classical signal could move from one to the other. This particular twist was not used in this experiment, but it is possible. Instead, we had the sit

Hello Dr.Chinese, I have actually read a lot of your web-site papers on entanglement mechanics. There is a loophole discovered by Jack Sarfatii {PhD} and further derived by Kieth Kremner (a computer scientist), had also proved for him in a different mathematical method which showed there is a loophole in light-bound messages. Apparently the loophole allows for superluminal communication of signals between Alice and BoB.

Hello Dr.Chinese, I have actually read a lot of your web-site papers on entanglement mechanics. There is a loophole discovered by Jack Sarfatii {PhD} and further derived by Kieth Kremner (a computer scientist), had also proved for him in a different mathematical method which showed there is a loophole in light-bound messages. Apparently the loophole allows for superluminal communication of signals between Alice and BoB.

I will write out the method if asked.

Hi Meselwulf,

Happy if anything I have written has been of interest in any way.

I am familiar with Jack's name, but not familiar with anything he has done which really relates to this discussion (I have had discussions with him many years ago). Certainly if you think there is some technique that applies here, I would be interested, but around here I think citing him is not going to get you very far. I am trying to be kind, I believe you have only started posting relatively recently. Speculative science is not welcome here. And this thread in particular, as I have cited experimental work which is fully in keeping with generally accepted theory. If you feel the need to take us on a tangent, please start a new thread and be mindful of forum rules - we have moderation here. (I may sound like I am being a prude, but believe me I am not.)

So basically I am saying there is no possibility of superluminal signalling, not that such is a subject for this thread. This thread is pointing out that consistent with the predictions of QM, it is possible to entangle particles that have not existed at the same time.

Jack Sarfatti is a well known new age fringe physics guy who was part of the infamous Fundamental Fysiks group in Berkeley. His stuff typically does not make it through peer review in acceptable journals and abstracts starting with "I conjecture that the intrinsic conscious mental field is analogous to a hologram described by a low frequency effective field nonlinear dissipative (non-unitary) local macroquantum coherent c-number multi-component order parameter." (Retrocausality and Signal Nonlocality in Consciousness and Cosmology, Journal of Cosmology, 14 (2011)) make it clear why that is the case.

The mere fact that someone claims to see a loophole does not actually make it a loophole. As DrChinese already pointed out, this is definitely not a forum devoted to fringe physics and there are really good reasons why that is the case.

Back to topic: I do not think one has to be deeply impressed by the results if one already knows and understands entanglement swapping. These results are the logical next step. Nevertheless the results are a very cool and clear experimental demonstration following the predictions of QM.

Sorry for the broad question, perhaps, but does this have anything to tell us about space-time boundaries, e.g., in cosmology or black-hole physics? I know for example that accelerating cosmic expansion means areas outside your hubble volume can never be reached, but would entanglement between such vastly separated particles be similar to this in a way? (The other example would be separation by a B.H. event horizon.)

Continuing in line with the experiment/paper at hand, would the entanglement allow you to 'know' things about an outgoing particle that was not-even in principle observable? I can imagine I'm touching on a number of topics here (like B.H. information loss, etc.) but it seems like your information would be becoming useless.

Experimental realization of that setup shows that photons 1 and 4 violate a Bell Inequality, demonstrating their entanglement. Whoever said QM was a "strange theory" was certainly not exaggerating! A future context affects the past (since the decision to entangle 1 and 4 is made after 4 is detected) !!

Thanks for the heads up on this paper, DrChinese!

Is that a typo in the last sentence? The decision to entangle 1 and 4 is made after 1 is detected.

Is that a typo in the last sentence? The decision to entangle 1 and 4 is made after 1 is detected.

Great eye as always, RUTA! I should have written as you say, the decision to entangle 1 and 4 is made after 1 is detected. That is the actual experiment. Theoretically, you could make the decision to entangle AFTER both 1 and 4 are detected, and that has been done in other experiments. And also, you could also theoretically have the 1 and 4 photons positioned in such a way that the detection of one was not local to the detection of the other.

Of course, the real thing about this experiment (to me anyway) is the idea that the measurement context spans both space and time. When you allow that to happen, there is no need to have any FTL influences even though the net effect appears to be FTL.

This experiment is quite a strong argument for a blockworld type interpretation of QM, in case you know anyone who is selling those.

But that obviously wouldn't fly in a classical regime - how can you entangle a particle that doesn't exist after the fact?

Perhaps –

The photon (photon 1 in the above example) before "ceasing to exist" i. e. before being destroyed in its present form, transfers the entanglement to the entity with which it interacts. Thus photon 4 is entangled not with photon 1 but with the particle (let's call it photon 5) photon1 interacted with in the process of being destroyed.

We can look at entanglement and particle as two separable entities and that entanglement is transferable between particles/entities.

Particles can cease to exist but entanglement can remain. One of the original wedded partner can cease to exist (say due to old age) but the marriage can remain.

On a separate note: the photon, in a deeper sense, can never cease to exist.(?)

Photons 1 and 2 are created as an entangled pair. The polarization of photon 1 is measured, so it no longer exists. Later, photons 3 and 4 are created as an entangled pair. Even later, the entanglement of photon 2 is swapped with photon 3. This means that photon 4 is now entangled with photon 1, even though photon 1 no longer exists!

Experimental realization of that setup shows that photons 1 and 4 violate a Bell Inequality, demonstrating their entanglement.

I'm failing to see why this is surprising. I thought the whole point of entanglement was that the pair shared a single state (for the property under consideration)? If anything, it's the possibility of entanglement swapping that's surprising and weird to me but given swapping, I don't see why the above shouldn't follow.

It's not at all surprising, taking QM at face value which any reasonable person must (consciousness bs and such notwithstanding). What is required to label it as surprising is to attempt to view it from within certain flavors of classical interpretations. It is interesting nonetheless because it drives a deeper experimental, not just theoretical (even it it is QED), stake in the heart of these specific kinds of interpretations.